1. Field of Invention
The present invention relates to an illumination optical system which uniformizes in-plane illuminance distribution of the light emitted from a light source, and to a projector having such an optical system.
2. Description of Related Art
Currently, for liquid crystal projectors, so-called three-plate type reflective liquid crystal projectors, which use three reflective liquid crystal panels, is known. The three-plate type reflective liquid crystal projector separates the light emitted from a light source into lights of three colors, that is, three primary colors, red (R), green (G), and blue (B) by a color separation system. Then the separated three color lights illuminate three reflective liquid crystal panels for each color light, the three primary colors modulated by each of the reflective liquid crystal panels are synthesized, and the color image obtained by the synthesis is projected, in an enlarged form, onto a screen by the projection lens.
In the above-described reflective liquid crystal projectors, miniaturization of the apparatus is considered to be important, so that optical elements having dichroic planes disposed at 45xc2x0 to the optical axis are often used for color separation and color synthesis. However, these projectors have a problem in that chrominance non-uniformity often occurs: by the polarization dependency of light separation characteristics of dichroic planes, thereby making it difficult to improve image quality.
Under this circumstance, several optical systems, which seldom cause chrominance non-uniformity in consideration of the characteristics of dichroic planes, and thus realize image quality improvement, have been proposed. For example, in Japanese Unexamined Patent Application Publication Nos. 7-84218 and 11-64794, optical systems have been proposed, in which a polarized beam splitter having a wavelength selection retardation film and a light separation function are used instead of dichroic planes for light separation. However, there remains a problem in that with a polarized beam splitter having a wavelength selection retardation film and light separation function, it is difficult to realize light separation which varies steeply, and that the cost becomes too high.
Accordingly, it is an object of the present invention to provide an illumination optical system which efficiently generates illumination light having specific color light with the polarization direction which is different by 90xc2x0 to the polarization direction of the other color light, so that such illumination light can provide the illuminated area with uniform illuminance distribution. Furthermore, it is another object of the present invention to provide a projector to which such an illumination optical system is applied, thus in which the polarization dependency of dichroic planes which constitute the color separation/synthesis optical system is reduced, and high quality projection image is displayed.
According to a first aspect of the present invention, there can be provided an illumination optical system including a luminous flux division optical element which divides the light from a light source into a plurality of partial luminous fluxes and collects each partial luminous flux, a color light separation optical element which separates each of the partial luminous fluxes into first color partial luminous flux and second color partial luminous flux, and emits the first color partial luminous flux and the second color partial luminous flux in different directions with each other or in a direction parallel to each other, a polarization change element which includes a polarization beam splitter array in which a plurality of polarization separation films and a plurality of reflective films are arranged alternately, and a polarization direction rotation element which is disposed either at a position where light transmitted through the polarization separation film is emitted or at a position where light reflected by the reflective film is emitted. The first color partial luminous flux incident on the polarization separation film is uniformed in a first polarization direction, and the second color partial luminous flux incident on the reflective film is uniformed in a second polarization direction. The invention can also include a transmission optical element which is disposed either at an incident side or at an emitting side of the polarization change element, and transmits an image formed by the luminous flux division optical element to an illuminated area, and a superposition optical element for superposing partial luminous flux emitted from the polarization change element at the illuminated area.
With this arrangement, first, the light from a light source is divided into a plurality of partial luminous fluxes and collected by the luminous flux division optical element, and each of the partial luminous fluxes is separated into the first color partial luminous flux and the second color partial luminous flux. The separated first color light and second color light enter the polarization change element having the polarization beam splitter array and the polarization direction rotation element, and are transformed into the first color partial luminous flux and second color partial luminous flux, each luminous flux having desired polarization state for each color light. Here, the polarization beam splitter array has a structure in which a plurality of pairs of polarization separation film and reflective film are arranged, and the polarization direction rotation elements are disposed at the emitting side of the polarization beam splitter array by selecting the positions corresponding to the positions of the polarization separation films or those of the reflective films.
For example, the polarization direction rotation elements are disposed only at the emitting side of the polarization separation films. Accordingly, of the first color partial luminous flux and the second color partial luminous flux, one enters a polarization separation film, and the other enters a reflective film selectively. Furthermore, the first color partial luminous flux and second color partial luminous flux are individually separated at the polarization beam splitter array into two kinds of polarization luminous fluxes, that is, a partial luminous flux having the first polarization direction which allows transmission of the polarization separation film and a partial luminous flux having the second polarization direction which is reflected by the polarization separation film.
Of the two kinds of polarization luminous fluxes, the polarization direction of one of the polarization luminous fluxes is rotated about 90xc2x0 by passing through a retardation film (polarization direction rotation element) such as a xcex/2 wavelength plate. Since the first color partial luminous flux and the second color partial luminous flux enter different films (polarization separation film and reflective film), respectively, the first color partial luminous flux and the second color partial luminous flux are uniformed in different polarization directions such that the first color partial luminous flux is uniformed in a first polarization direction and the second color partial luminous flux is uniformed in a second polarization direction.
For example, every first color partial luminous flux is arranged in S polarization light and every second color partial luminous flux is arranged in P polarization light. Then these partial luminous fluxes are superposed at the illuminated area through the superposition optical element. The transmission optical element has a function to transmit each partial luminous flux to the illuminated area The transmission optical element can be disposed either at the incident side or at the emitting side of the polarization change element. If the transmission optical element is disposed at the incident side of the polarization change element, each partial luminous flux becomes possible to enter the polarization change element at a predetermined angle, thereby making it easy to improve the polarization separation function of the polarization separation film. Thus, on the point of illumination efficiency, it is more advantageous to dispose the transmission optical element at the incident side of the polarization change element. On the other hand, if the transmission optical element is disposed at the emitting side of the polarization change element, it is possible to make one-piece optical element including the superposition optical element and the transmission optical element by implementing the function of the superposition optical element in the superposition optical element. It is therefore more advantageous to dispose the transmission optical element at the emitting side of the polarization change element when the number of parts needs to be reduced. As described above, according to the first aspect of the present invention, non-polarized light from a light source is transformed into polarization luminous flux which has a uniform polarization direction for each color light in advance. Thus, it is possible to reduce the polarization dependency of the optical elements, such as dichroic prisms and polarization beam splitters, which are disposed at more downstream side of the light path than the illumination optical system. It is therefore also possible to increase illumination efficiency.
Further, according to a second aspect of the present invention, there can be provided an illumination optical system including a color light separation optical element which separates the light from a light source into a first color light and a second color, light, and emits the first color light and the second color light in different directions with each other or in a direction parallel to each other, a luminous flux division optical element which divides the first color light into a plurality of the first color partial luminous flux, divides the second color light into a plurality of the second color partial luminous flux, and collects each of the partial luminous fluxes. The invention can further include a polarization change element which includes a polarization beam splitter array in which a plurality of polarization separation films and a plurality of reflective films are arranged alternately, and a polarization direction rotation element which is disposed either at a position where the light transmitted through the polarization separation film is emitted or at a position where the light reflected by the reflective film is emitted. The first color partial luminous flux incident on the polarization separation film is uniformed into polarized light having a first polarization direction, and the second color partial luminous flux incident on the reflective film is uniformed into polarized light having a second polarization direction. The invention can further include a transmission optical element which is disposed either at an incident side or at an emitting side of the polarization change element, and transmits an image formed by the luminous flux division optical element to an illuminated area, and a superposition optical element for superposing partial luminous flux emitted from the polarization change element at the illuminated area.
With this arrangement, first, the light from a light source is separated into the first color light and the second color light by the color light separation optical element. The first color light and the second color light are individually divided into a plurality of partial luminous fluxes and collected by the luminous flux division optical element. Specifically, the first color light is divided into the first color partial luminous fluxes, and the second color light is divided into the second color partial luminous fluxes. Each of these partial luminous fluxes enters the polarization change element having the polarization beam splitter array and the polarization direction rotation element, and is transformed into the first color partial luminous flux and second color partial luminous flux, each partial luminous flux having desired polarization state for each color light. Here the structure of the polarization beam splitter array is the same as that of the illumination optical system of the first aspect described above. Accordingly, of the first color partial luminous flux and the second color partial luminous flux, one enters into a polarization separation film, and the other enters into a reflective film. The subsequent operation is the same as that of the previous illumination optical system of the first aspect.
In the case of illumination optical system of the second aspect, non-polarized light from a light source is transformed into polarization luminous flux which has a uniform polarization direction for each color light in advance, thus it is possible to obtain the same effect as that of the first aspect. Furthermore, in the illumination optical system of the second aspect, since the color light separation optical element is disposed between the light, source and the luminous flux division optical system, highly parallel light can be entered into the color light separation optical element. Thus in the color light separation optical element, separation of color light can be performed much more efficiently without fail. In this regard, in the illumination optical system of the second aspect, as is the case with the illumination optical system of the first aspect, the transmission optical element can be disposed either at the incident side or at the emitting side of the polarization change element.
Furthermore, according to a third aspect of the present invention, there can be provided an illumination optical system including a luminous flux division optical element which divides light from a light source into a plurality of partial luminous fluxes and collects each of the partial luminous fluxes, a color light separation optical element which separates the each partial luminous flux into first color partial luminous flux and second color partial luminous flux, and emits the first color partial luminous flux and the second color partial luminous flux in different directions with each other or in a direction parallel to each other, and a polarization change element which includes a polarization beam splitter array in which a plurality of polarization separation films are arranged at a predetermined interval, and polarization direction rotation elements which are arranged at the predetermined interval and are disposed at an emitting side of the polarization beam splitter array. The first color partial luminous flux incident on an incident side end plane of the polarization separation film where the polarization direction rotation elements are not disposed at an emitting side of the film and transmitting through the polarization separation film, and the first color partial luminous flux reflected by the polarization separation film and then reflected by an adjacent polarization separation film once again and transmitting through the polarization direction rotation elements are uniformed in a first polarization direction to be emitted, while the second color partial luminous flux incident on an incident side end plane of the polarization separation film where the polarization direction rotation elements are disposed at an emitting side of the film and transmitting through the polarization separation film and then transmitting through the polarization direction rotation element, and the second color partial luminous flux reflected by the polarization separation film and then reflected by an adjacent polarization separation film once again are uniformed in a second polarization direction to be emitted. The invention can also include a transmission optical element which is disposed either at an incident side or at an emitting side of the polarization change element, and transmits an image formed by the luminous flux division optical element on an illuminated area, and a superposition optical element for superposing partial luminous flux emitted from the polarization change element at the illuminated area.
With this arrangement, first, the light from a light source is divided into a plurality of partial luminous fluxes and collected by the luminous flux division optical element, and each of the partial luminous fluxes is separated into the first color partial luminous flux and the second color partial luminous flux by the luminous flux division optical element. The separated first color light and second color light enter the polarization change element having the polarization beam splitter array and the polarization direction rotation element, and are transformed into the first color partial luminous flux and second color partial luminous flux, each partial luminous flux having desired polarization state for each color light. Here, the polarization beam splitter array has a structure in which a plurality of polarization separation films are arranged, and polarization direction rotation elements are disposed at the emitting side of the polarization beam splitter array by selecting the positions corresponding to the specific positions of the polarization separation films. For example, the polarization direction rotation elements are disposed only at the emitting side of every other polarization separation films. Now, suppose the polarization separation film which is provided with the polarization direction rotation element at the emitting side is referred to as a polarization separation film A, and the polarization separation film which is not provided with the polarization direction rotation element at the emitting side is referred to as a polarization separation film B for convenience sake. Accordingly, of the first color partial luminous flux and the second color partial luminous flux, one enters the polarization separation film B, and the other enters the polarization separation film A, selectively.
In the same manner as the polarization separation film described above, the polarization separation films A and B separate the entered partial luminous flux into partial luminous flux having the first polarization direction which allows transmission and partial luminous flux having the second polarization direction which is reflected. The partial luminous flux which has transmitted through the polarization separation film B is emitted from the polarization change element as the partial luminous flux having the first polarization direction. Also, the partial luminous flux which has been reflected by the polarization separation film B is the partial luminous flux having the second polarization direction, is reflected once again by the adjacent polarization separation film A, and then is rotated about 90xc2x0 by passing through a retardation film (polarization direction rotation element) such as a xcex/2 wavelength plate. Then the partial luminous flux is emitted from the polarization change element as the partial luminous flux having the first polarization direction. On the other hand, the partial luminous flux which has transmitted through the polarization separation film A is the partial luminous flux having the first polarization direction, is rotated about 90xc2x0 by passing through a retardation film such as a xcex/2 wavelength plate, and is emitted from the polarization change element as the partial luminous flux having the second polarization direction. Furthermore, the partial luminous flux which has been reflected by the polarization separation film A is reflected once again by the adjacent polarization separation film B, and then is emitted from the polarization change element as the partial luminous flux having the second polarization direction.
Since the first color partial luminous flux and the second color partial luminous flux enter the polarization separation film distinguished by the existence of the polarization direction rotation element, the first color partial luminous flux and the second color partial luminous flux are uniformed in different polarization directions such that the first color partial luminous flux is uniformed in a first polarization direction and the second color partial luminous flux is uniformed in a second polarization direction.
For example, the first color partial luminous fluxes are all arranged in S polarization light and the second color partial luminous fluxes are all arranged in P polarization light. Then these partial luminous fluxes are superposed at the illuminated area through the superposition optical element. The subsequent operation is the same as that of the previous illumination optical system of the first aspect.
In the illumination optical system of the third aspect, as compared with the illumination optical systems of the first aspect and the second aspect, of the first color partial luminous flux and second color partial luminous flux within the polarization change element, it is possible with ease to make the difference of the light path length smaller between the partial luminous flux having the shortest light path and the partial luminous flux having the longest light path. Thus, in the illuminated area, it is possible to make the magnification factor of the first color partial luminous flux and the magnification factor of the second color partial luminous flux the same. Consequently, illumination efficiency can be improved. Also, while the polarization beam splitter arrays in the above-described illumination optical systems of the first and second aspects have the polarization separation films and the reflective films, the polarization beam splitter array in the illumination optical system,of the third aspect has only the polarization separation films. Thus the structure of the polarization beam splitter array is simple, and it is therefore easy to be manufactured.
Moreover, according to a fourth aspect of the present invention, there can be provided an illumination optical system including a color light separation optical element which separates light from a light source into first color light and second color light, and emits the first color light and the second color light in different directions with each other or in a direction parallel to each other, a luminous flux division optical element which divides the first color light into a plurality of the first color luminous fluxes, divides the second color light into a plurality of the second color luminous fluxes, and collects each of the partial luminous fluxes, and a polarization change element which includes a polarization beam splitter array in which a plurality of polarization separation films are arranged at a predetermined interval, and a polarization direction rotation elements which are arranged at the predetermined interval and are disposed at an emitting side of the polarization beam splitter array. The first color partial luminous flux incident on an incident side end plane of the polarization separation film where the polarization direction rotation elements are not disposed at an emitting side of the film and transmitting through the polarization separation film, and the first color partial luminous flux reflected by the polarization separation film and then reflected by an adjacent polarization separation film once again and transmitting through the polarization direction rotation element are uniformed in a first polarization direction to be emitted, while the second color partial luminous flux incident on an incident side end plane of the polarization separation film where the polarization direction rotation element is disposed at an emitting side of the film and transmitting through the polarization separation film and then transmitting through the polarization direction rotation element, and the second color partial luminous flux reflected by the polarization separation film and then reflected by an adjacent polarization separation film once again are uniformed in a second polarization direction to be emitted. The invention can also include a transmission optical element which is disposed either at an incident side or at an emitting side of the polarization change element, and transmits an image formed by the luminous flux division optical element on an illuminated area, and a superposition optical element for superposing the partial luminous fluxes emitted from the polarization change element at the illuminated area.
With this arrangement, first, the light from a light source is separated into the first color light and the second color light by the color light separation optical element. The first color light and the second color light are individually divided into a plurality of partial luminous fluxes and collected by the luminous flux division optical element. Specifically, the first color light is divided into the first color partial luminous fluxes, and the second color light is divided into the second color partial luminous fluxes. Each of these partial luminous fluxes enters the polarization change element having the polarization beam splitter array and the polarization direction rotation element, and is transformed into the first color partial luminous flux and second color partial luminous flux, each partial luminous flux having desired polarization state for each color light. Here the structure of the polarization beam splitter array is the same as that of the illumination optical system of the third aspect described above. Accordingly, the first color partial luminous flux enters the polarization separation film B, and the second color partial luminous flux enters the polarization separation film A individually by selecting positions. The subsequent operation is the same as that of the illumination optical system of the third aspect.
In the illumination optical system of the fourth aspect, in the same manner as the illumination optical system of the third aspect, as compared with the illumination optical systems of the first aspect and the second aspect, of the first color partial luminous flux and second color partial luminous flux within the polarization change element, it is possible to make the light path length difference smaller between the partial luminous flux having the shortest light path and the partial luminous flux having the longest light path. Thus, in the illuminated area, it is easily possible to make the magnification factor of the first color partial luminous flux and the magnification factor of the second color partial luminous flux the same. Consequently, illumination efficiency can be improved. Also, in the same manner as the illumination optical system of the third aspect, the structure of the polarization beam splitter array is simple, thus it is easy to be manufactured.
The color light separation optical element to be used in the illumination optical system of the first, the second, the third, and the fourth aspects can be constructed of two mirrors, one optical part having two mirrors, a reflective hologram, or a transmissive hologram.
When constructing the color light separation optical element by two mirrors, the first mirror may be set to a dichroic mirror for performing color separation, and the second mirror may be set to perform a reflective mirror. In general, dichroic mirrors and reflective mirrors have high reflection factor. Thus if such mirrors are used in the structure, it becomes possible to separate a color light with high efficiency without fail. Here, it is possible to construct a reflective mirror not only using a general reflective mirror which is formed with metal film such as aluminum, but also using a dichroic mirror which reflects specific color light. With this arrangement, unnecessary light (for example, infrared light ultraviolet light, and specific color light such as yellow light) can be removed from the illumination light by the color light separation optical element. Thus when using these illumination optical systems for a projector, it is possible to improve the reliability of the light modulation device used for the projector, and to improve image quality of the projection image. In this regard, the function of the second mirror is to reflect a specific color light which is transmitted through the first mirror, thus the second mirror is not necessarily a dichroic mirror. However, when using a dichroic mirror, it is easy to obtain higher reflection factor as compared with a general reflective mirror, and thus it is convenient to increase light utilization efficiency in the color light separation optical element.
Furthermore, when using two mirrors, it is preferable to dispose the first mirror and the second mirror as follows:
(1) The first mirror and the second mirror are not parallel to each other, the first mirror is disposed at an angle of 45xc2x0 to the optical axis of the light source, and the second mirror is disposed at an angle of (45+xcex1)xc2x0 to the optical axis of the light source.
(2) The first mirror and the second mirror are not parallel to each other, the first mirror is disposed at an angle of (45+)xc2x0 to the optical axis of the light source, and the second mirror is disposed at the angle of 45xc2x0 to the optical axis of the light source.
(3) The first mirror and the second mirror are not parallel to each other, the first mirror is disposed at an angle of (45+xcex2)xc2x0 to the optical axis of the light source, and the second mirror is disposed at the angle of (45xe2x88x92xcex2)xc2x0 to the optical axis of the light source.
(4) The first mirror and the second mirror are parallel to each other at a predetermined distance, and are disposed at an angle of 45xc2x0 to the optical axis of the light source.
Particularly, when disposing the mirrors as in the cases (3) and (4), a color light can be separated symmetrically with respective to a predetermined axis, and thus it is preferable for simplifying the structure of the transmission optical element.
Also, in the above cases (1) to (3), the function of the color light separation optical element is to make the directions of the luminous fluxes which are emitted toward the polarization change element different between the first color partial luminous flux and the second color partial luminous flux. Thus in order to realize this function, the first mirror and the second mirror may be disposed not parallel to each other, and thus the disposition angles of the first mirror and the second mirror are not limited to the examples described above. However, the optical characteristic of the transmission optical element needs to be set appropriately in response to an incident angle of the color light to the transmission optical element.
Next, a description will be given of the case where the color light separation optical element is composed of an optical part having two mirrors. For an optical part having two mirrors, examples are as follows:
(A) An optical part including a plate translucent member, a dichroic mirror disposed on one of two planes opposed with each other of the translucent member, and a reflective mirror disposed on the other of the planes.
(B) An optical part including a plate translucent member, a rectangular prism fixed firmly on one of two planes opposed with each other of the translucent member, a reflective mirror disposed on the other of the planes, and a dichroic mirror disposed between the translucent member and the rectangular prism.
(C) An optical part including a plate translucent member, a plurality of small size rectangular prisms fixed firmly on one of two planes opposed with each other of the translucent member, a reflective mirror disposed on the other of the planes, and a dichroic mirror disposed between the translucent member and the rectangular prisms.
When the color light separation optical element is composed of one optical part like this, assembling the optical system can be made easily. Also, if one optical parts such as (B) or (C) is used, light enters the dichroic mirror through a rectangular prism having a refraction factor greater than 1. Thus the incident angle of the light on the dichroic mirror is narrowed, so that the light separation characteristic of the dichroic mirror is enhanced, and light path shift can be eliminated. Furthermore, if one optical part such as (C) is used, the prism part can be miniaturized, and thus the color light separation optical element can be miniaturized and the weight thereof can be saved. In this regard, it is possible to construct a reflective mirror not only using a general reflective mirror which is formed with metal film such as aluminum, but also using a dichroic mirror which reflects specific color light, and the above-described effect can be obtained. The function of the second mirror is to reflect a specific color light which has been transmitted through the first mirror, thus the second mirror is not necessarily a dichroic mirror. However, when using a dichroic mirror, it is easy to obtain a higher reflection factor as compared with a general reflective mirror, and thus it is convenient to increase light utilization efficiency in the color light separation optical element.
Furthermore, in the optical parts (A) to (C), one of the planes, on which a dichroic mirror is disposed and the other of the planes, on which a reflective mirror is disposed, are preferably arranged as follows:
(a) The one of the planes and the other of the planes are not parallel to each other, the one of the planes is disposed at an angle of 45xc2x0 to the optical axis of the light source, and the other of the planes is disposed at an angle of (45xe2x88x92xcex1)xc2x0 to the optical axis of the light source.
(b) The one of the planes and the other of the planes are not parallel to each other, the one of the planes is disposed at an angle of (45+xcex1)xc2x0 to the optical axis of the light source, and the other of the planes is disposed at an angle of 45xc2x0 to the optical axis of the light source.
(c) The one of the planes and the other of the planes are not parallel to each other, the one of the planes is disposed at an angle of (45+xcex2)xc2x0 to the optical axis of the light source, and the other of the planes is disposed at an angle of (45xe2x88x92xcex2)xc2x0 to the optical axis of the light source.
(d) The one of the planes and the other of the planes are parallel to each other with a predetermined distance therebetween, and are individually disposed at an angle of 45xc2x0 to the optical axis of the light source.
Particularly, when disposing the mirrors as in the cases (c) and (d), a color light can be separated symmetrically with respective to a predetermined axis, and thus it is preferable for simplifying the structure of the transmission optical element.
Also, in the above cases (1) to (3), the function of the color light separation optical element is to make the direction of the first color partial luminous flux different from that of the second color partial luminous flux which are emitted toward the polarization change element. Thus in order to realize this function, the one of the planes and the other of the planes may be disposed not parallel to each other, and thus the disposition angles of the one of the planes and the other of the planes are not limited to the examples described above. However, the optical characteristic of the transmission optical element needs to be set appropriately in response to an incident angle of the color light to the transmission optical element.
Finally, a description will be given of the case where the color light separation optical element is made of a reflective hologram element or a transmissive hologram element. In this case, the color light separation optical element can be constructed of one plate hologram, thus the number of parts of the color light separation optical element can be reduced and the illumination optical system can be miniaturized and the weight thereof can be saved.
The luminous flux division optical element to be used for an illumination optical system can be constructed of a lens array, a mirror array, a light guiding rod having four reflection planes, and so forth. If a mirror array is used, the cost becomes lower than the case of using a lens array or a light guiding rod. Also, if a mirror array or a light guiding rod is used, spherical aberration, which always accompanies with a lens array, does not occur. Thus light condensing is enhanced, and illumination efficiency can be improved.
Also, in the illumination optical system of the present invention, it is further preferable to dispose a dichroic filter array in order to block unnecessary incident color light on an incident side of the polarization beam splitter array. When disposing such a dichroic filter array, even if a color light separation optical element having a relatively higher incident angle dependency in light separation characteristic, unnecessary color light is prevented from entering into the polarization beam splitter array. Thus the first color light and the second color light can be separated without fail. In this regard, when disposing the transmission optical element at the incident side of the polarization change element, the dichroic filter array can be disposed not only between the transmission optical element and the polarization change element, but also at the incident side of the transmission optical element.
Furthermore, in the illumination optical system of the present invention, the color light separation optical element preferably has a color separation characteristic that green light is separated from red and blue light. With this arrangement, it becomes easy to optimize the selection characteristic of the green light of the color light separation optical element. Thus if a illumination optical system having such a structure is applied to a projector, it becomes easier to enhance the contrast and utilization efficiency of green light, and it becomes possible to display a projection image with high contrast and brightness.
Moreover, using the illumination optical system described above, when constructing a projector having a light modulating device for modulating the light emitted from the illumination optical system and a projection lens for projecting the modulated light by the light modulating device, it is possible to reduce the polarization dependency of the optical elements disposed at more downstream side of the light path than the illumination optical system. Thus it becomes possible to improve the image quality and brightness of the projection image.
Particularly, the illumination optical system of the present invention is preferably applied to the following projector:
(I) A projector including an illumination optical system described above, a first reflective light modulation device for modulating the first color light emitted from the illumination optical system, a second reflective light modulation device for modulating the third color light included in the second color light emitted from the illumination optical system, a third reflective light modulation device for modulating the fourth color light included in the second color light emitted from the illumination optical system, and a polarization beam splitter for separating light emitted from the illumination optical system into the first color light and the second color light. The projector can also include a projection lens which includes a color light separation/synthesis element for separating the second color light into the third color light and the fourth color light and also for synthesizing light emitted from the second reflective light modulation device and light emitted from the third reflective light modulation device to emit light to the polarization beam splitter, wherein light selected by the polarization beam splitter out of light emitted from the first reflective light modulation device and light emitted from the color light separation/synthesis element is projected.
(II) A projector including an illumination optical system described above, a first reflective light modulation device for modulating the first color light included in light emitted from the illumination optical system, a second reflective light modulation device for modulating the third color light included in the second color light emitted from the illumination optical system, a third reflective light modulation device for modulating the fourth color light included in the second color light emitted from the illumination optical system, first to fourth polarization beam splitters, a first wavelength selection retardation film disposed between the first polarization beam splitter and the third polarization beam splitter, and a second wavelength selection retardation film disposed between the third polarization beam splitter and the fourth polarization beam splitter. The projector can also include a projection lens for projecting light emitted from the fourth polarization beam splitter, wherein the first polarization beam splitter separates light emitted from the illumination optical system into a first color light and a second color light, the second polarization beam splitter leads the first color light separated by the first polarization beam splitter into the first reflective light modulation device, and also leads the first color light modulated by the first reflective light modulation device into the fourth polarization beam splitter, the first wavelength selection retardation film rotates about 90xc2x0 only a polarization direction of the third color light out of the third color light and the fourth color light included in the second color light separated by the first polarization beam splitter, the third polarization beam splitter leads the third color light and the fourth color light emitted from the first wavelength selection retardation film into the second reflective light modulation device and the third reflective light modulation device, and also leads the third color light and the fourth color light modulated by the second reflective light modulation device and the third reflective light modulation device into the second wavelength selection retardation film, the second wavelength selection retardation film rotates about 90xc2x0 only a polarization direction of the third color light out of the third color light and the fourth color light emitted from the third polarization beam splitter, and the fourth polarization beam splitter synthesizes the first color light emitted from the second polarization beam splitter, and the third color light and the fourth color light emitted from the second wavelength selection retardation film, and emits it toward the projection lens.
(III) A projector including an illumination optical system described above, a color separation optical system for separating light emitted from the illumination optical system into first color light, second color light, and third color light, a first transmissive light modulation device for modulating the first color light separated by the color separation optical system in response to an image signal, a second transmissive light modulation device for modulating the second color light separated by the color separation optical system in response to an image signal, a third transmissive light modulation device for modulating the third color light separated by the color separation optical system in response to an image signal, a color synthesis optical system for synthesizing the first color light, the second color light, and the third color light which have been modulated by the first transmissive light modulation device, the second transmissive light modulation device, and the third transmissive light modulation device, respectively, and a projection lens for projecting light synthesized by the color synthesis optical system.
When constructing a projector as described in (I), (II), and (III), the polarization dependency of the light separation characteristic of a dichroic mirror, a dichroic prism, and a polarization beam splitter array can be reduced. Thus, it is possible to achieve high quality and high brightness of the projection image, as well as cost reduction of the optical system which performs color light separation and synthesis. Also, in a projector having a structure as described in (II), each color light reaches the projection lens entirely through two polarization beam splitters, and thus the contrast of the projection image of the projector can be further enhanced. In this regard, the first and the fourth polarization beam splitters can be replaced with a dichroic mirror or a dichroic prism, and the cost reduction can be achieved in this case. Furthermore, in the illumination optical system of the present invention, of the three color lights, that is, the first color light, the second color light, and the third color light, one color light can be emitted with having a different polarization state from those of the other two color lights. Usually, in a so-called three-plate type projector, which includes three transmissive light modulation devices for modulating the first color light, the second color light, and the third color light, respectively, and the color synthesis optical system for synthesizing the first color light, the second color light, and the third color light, which have been modulated by the respective transmissive light modulation devices, in order to improve the synthesis efficiency of the color light in the color synthesis optical system, a xcex/2 wavelength plate is disposed just in front of or just at the back of the transmissive light modulation device. Accordingly, the polarization state of at least one color light of the incident light onto the color synthesis optical system differs from the polarization states of the other color lights. However, when using the illumination optical system of the present invention, the xcex/2 wavelength plate used for such a purpose can be omitted. Consequently, cost reduction can be achieved.
For example, when the illumination optical system has a structure in which green light is emitted as S polarization light, and blue and red light is emitted as P polarization light, it is unnecessary to dispose a xcex/2 wavelength plate just in front of or just at the back of the transmissive light modulation device. Also, when the illumination optical system has a structure in which green light is emitted as P polarization light, and blue and red light is emitted as S polarization light, the same number of xcex/2 wavelength plates becomes necessary for each transmissive light modulation device just in front of or just at the back of all, that is, the first to the third, transmissive light modulation devices. In a light path for each color, the same number of xcex/2 wavelength plates are disposed, thus chrominance non-uniformity can be reduced.
Furthermore, depending on the display characteristic of the transmissive light modulation device, the polarization state of the incident light onto the transmissive light modulation device may be limited. For example, when green light is entered as S polarization light, and blue and red light is entered as P polarization light into the transmissive light modulation device, the structure of the projector described in (III) is effective,